Faking SM Photons by Displaced Dark Photon Decays

TL;DR

This paper investigates how displaced dark photon decays can mimic photons at the LHC, affecting measurements and interpretations of new physics signals, especially in resonance spin determination and cross section comparisons.

Contribution

It provides a detailed analysis of dark photon fake rates based on detector geometry and explores implications for resonance properties and future photon identification strategies.

Findings

Dark photon decays can fake photons with a probability of order one.

Detector effects influence the angular dependence of photon fake rates.

Dark photon scenarios can alter interpretations of resonance spin and cross section measurements.

Abstract

A light meta-stable dark photon decaying into collimated electron/positron pair can fake a photon, either converted or unconverted, at the LHC. The detailed object identification relies on the specifics of the detector and strategies for the reconstruction. We study the fake rate based on the ATLAS(CMS) detector geometry and show that it can be O(1) with a generic choice of parameters. Especially, the probability of being registered as a photon is angular dependent. Such detector effects can induce bias to measurements on certain properties of new physics. In this paper, we consider the scenario where dark photons in final states are from a heavy resonance decay. Consequently, the detector effects can dramatically affect the results when determine the spin of a resonance. Further, if the decay products from the heavy resonance are one photon and one dark photon, which has a large probability to fake a diphoton event, the resonance is allowed to be a vector. Due to the difference in detector, the cross sections measured in ATLAS and CMS do not necessarily match. Furthermore, if the diphoton signal is given by the dark photons, the SM $Z\gamma$ and $ZZ$ final states do not necessarily come with the $\gamma\gamma$ channel, which is a unique signature in our scenario. The issue studied here is relevant also for any future new physics searches with photon(s) in the final state. We discuss possible ways of distinguishing dark photon decay and real photon in the future.